An OLED (also referred to as organic electro luminescence (EL)) is for conducting a direct current on a fluorescent organic compound which is excited by application of an electric field, and thereby causing light emission of the compound. The OLED is drawing attention as a next-generation display device in terms of low-profileness, a wide view angle, and a wide gamut, etc. Whereas a driving method for the OLED includes a passive type and an active type, the active type is suitable for achieving a large-screen and high-definition display in light of aspects involving a material, a life, and crosstalks. This active type requires thin film transistor (TFT) driving, and a TFT array applying low-temperature polysilicon or amorphous silicon (a-Si) is drawing attention for this use.
For example, U.S. Pat. No. 5,179,345 discloses (FIG. 2) a conventional inspection method for a TFT array in a liquid crystal display (LCD). The method is configured to observe electric charges accumulated in a pixel capacitor with an integration circuit after writing a voltage in the pixel capacitor and thereby to inspect whether the voltage is written properly. Meanwhile, U.S. Pat. No. 4,983,911 discloses (FIGS. 1–3) a method to optically inspect writing in a pixel capacitor by use of a photoelectric element. Moreover, Japanese Unexamined Patent Publication No. 2002-108243 (FIG. 2) discloses a technique for inspecting whether a pixel unit operates normally prior to formation of an EL element, which is configured to perform inspection while connecting a power source to a common pixel electrode before patterning a pixel electrode.
Now, description will be made on comparison between an active matrix OLED (AMOLED) and an active matrix liquid crystal display (AMLCD). FIGS. 14A and 14B are diagrams for comparing and explaining pixel circuits in the AMOLED and the AMLCD. FIG. 14A shows a pixel circuit of the AMOLED and FIG. 14B shows a pixel circuit of the AMLCD. In FIG. 14B, the pixel circuit of a TFT array is formed by a TFT 310 which is connected to a data line (Data) and a gate line (Gate). Meanwhile, in the AMOLED shown in FIG. 14A, a driving TFT 302 which is an open drain driving transistor is connected adjacently to a pixel capacitor of a circuit similar to the one shown in FIG. 14B, and an OLED 301 being a light emitting element is connected to the driving TFT 302.
The pixel circuit is closed within a TFT array substrate in the case of the AMLCD shown in FIG. 14B. On the contrary, in the case of the AMOLED shown in FIG. 14A, the pixel circuit is not closed within the TFT array substrate because the OLED 301 does not exist therein. Accordingly, the driving TFT 302 is configured to be open-drain (or open-source) while a drain side (or a source side) thereof is directly connected to a pixel electrode. Although there are at least two TFTs in the pixel circuit of the TFT array substrate in the AMOLED, it is impossible to conduct an electric current on the driving TFT only by input and output from a panel interface terminal.
In this event, to reduce manufacturing costs of the current AMOLED panels, it is necessary to carry out a performance test on the independent TFT array and forward only a non-defective product to a subsequent process. It is desired to measure the performance of the driving TFT 302 prior to mounting the OLED 301 in the manufacture of the AMOLED panel due to the reasons that: a product yield of the current TFT arrays for the AMOLED panels is not sufficiently high; raw material costs of the OLED 301 are high; a process for forming the OLED 301 occupies relatively a long time in the entire manufacturing process; and so on.
However, in the independent TFT array, the OLED which is a constituent of the pixel circuit is not mounted as described above, and the driving TFT 302 is set to an open-drain (or open-source) state. That is, in the process prior to mounting the OLED, the OLED 301 indicated by broken lines in FIG. 14A is not connected and a normal circuit is not therefore established. Accordingly, it is not possible to inspect open/short defects in the driving TFT 302 only by input and output to/from the panel interface terminal.
U.S. Pat. No. 5,179,345 and U.S. Pat. No. 4,983,911 solely show the methods of inspecting the pixel circuit of the TFT array for the AMLCD as shown in FIG. 14B and do not possess a mechanism for supplying an electric current to the driving TFT 302 shown in FIG. 14A. As a result, it is not possible to perform open/short measurement of the driving TFT 302 set to the open-drain (or open-source) state by use of the known techniques.
Meanwhile, the technique disclosed in Japanese Unexamined Patent Publication No. 2002-108243 is capable of measuring unevenness in resistance components depending on pixels. However, this technique is not designed to perform inspection after patterning the pixel electrodes. Therefore, this technique cannot inspect defects which are attributable to patterning. Moreover, although this technique can inspect a defect of the driving TFT 302, the technique cannot specify a type of such a defect (whether the defect is an open defect or a short defect). As a result, this technique cannot count the number of bright points or dark points (dead points), which are defects of a display device after formation of the OLED 301, or obtain data corresponding to an evaluation standard set up by an inspector, for example.